Nonwoven fabrics bonded with interpolymer and process of preparing same



United States Patent 3,081,197 NONWOVEN FABRICS BONDED WITH INTER- POLYMER AND PROCESS OF PREPARING SAME Robert L. Adelman, Wilmington, Del., assignor to E. I. du Pont de Nemours and Company, Wilmington, Del., a corporation of Delaware No Drawing. Filed Sept. 10, 1959, Ser. No. 839,050 2 Claims. (Cl. 117-140) This invention relates to improvements in nonwoven fabrics of the type composed of loosely assembled fibers held together with a binder, and is more particularly concerned with modified polymers of vinyl acetate and the preparation of cellulosic nonwoven fabrics therewith.

Adequately bonded nonwoven fabrics .have potential advantages over woven fabrics for a large variety of uses. The base web of nonwoven fibers, to which the binder is applied, can be produced inexpensively and with low capital investment by carding, garnetting, airlaying, papermaking procedures or other known operations for which eificient automation is possible. The operation of bonding the fibers in place is much less expensive than spinning and weaving. Comparing the product with woven fabric, the bonded nonwovens can be made in a much greater range of thicknesses per unit weight, with more homogeneous structures, no unravelling tendency, and with greater water absorbency, porosity and resiliency when required.

Vinyl acetate polymers have been used for some time in the preparation of nonwoven fabrics, as these polymers have good adhesion to cellulosic and synthetic fibers, emulsions have good mechanical and chemical stability, and are of low cost. In order to obtain fabrics which are textile-like in quality, having drape, flexibility, and softness, the polyvinyl acetate resins have been softened with external plasticizers such as dibutyl phthalate before application to the web, or the vinyl acetate has been copolymerized with internally plasticizing comonomers such as the alkyl acrylates, the alkyl methacrylates, vinyl stearate, or the dialkyl fumarates or maleates. The internally plasticized copolymers impart to the resulting fabric somewhat improved wet strength and wet abrasion resistance over the use of externally pl-asticized homopolymers as binders, but still greater wet strength at high flexibility is desired for the widest use of these fabrics.

It is an object of this invention to provide new, improved polymeric binders for nonwoven fabrics. Another object is to provide relatively low cost binder-containing nonwoven fabrics of enhanced wet strength .and other properties. A further object is to provide a process for preparing such nonwoven fabrics. Other objects will become apparent from the specification and claims.

In accordance with this invention it has been found that these objects are achieved by means of post-curable, internally plasticized binders of modified polyvinyl ace- .tate used for the preparation of nonwoven fabrics by impregnating a loosely assembled mass of fibers with a dispersion of the polymeric binder in water or a solvent,

carbon atoms.

followed by moderate heating to dry the mass and cure the binder. The binders of this invention are interpolymers of at least 45% by weight of vinyl acetate, at least 10% of another polymerizable compound as an internal plasticizer and 0.3% to 12% of a post-curable comonomer, such as N-methylol acrylamide, glycidyl acrylate, glycidyl methacryl-ate and allyl glycidyl ether. These monomers all have the characteristic of readily polymerizing with vinyl acetate and being capable of further reaction in the dry or semi-dry state, after this polympound.

simplifies the preparation of nonwoven fabrics.

3,081,197 Patented Mar. 12, 1963 RC 0 O CH=OH:, CH=CH-CO OR, CHFC-CO OR and CHCO OR CH-C 0 OR wherein R is an alkyl or oxa-alkyl group of 4 to 18 The term oxa-alky denotes a group which contains ether oxygen in an otherwise hydrocarbon chain, as illustrated by the polyethylene glycols.

The binders of this invention are readily prepared by interpolymerization of the three monomers in aqueous dispersion systems, or in an organic solvent, in the same manner as has previously been described in the art for the formation of high molecular weight homopolymers of vinyl acetate, e.g., in aqueous dispersion systems containing a dispersing agent and a small amount of a con ventional polymerization catalyst such as ammonium persulfate, hydrogen peroxide, or other peroxygen com- Nonionic, anionic or cationic dispersing agents may be used, with the nonionic dispersers giving fabrics somewhat lower in wet strength compared to the other types. However, certain of the nonionic dispersing agents, such as polyvinyl alcohol, are useful in giving fabrics which are unlaminated when applied by padding fromaqueous dispersions, whereas ionic surfactants often give undesirable laminated fabrics when the binder is applied by padding. A particularly useful nonionic dispersing agent, which gives fabrics of high wet strength and, at the same time, gives unlaminated fabrics when applied by padding from aqueous dispersion, is hydroxyethyl polyvinyl alcohol.

The binders of this invention may be used to prepare nonwoven fabrics by a variety of methods known to the art which, in general, involve the impregnation of a loosely assembled mass of fibers with a dispersion of the binder in Water or solvent, followed by moderate heating to dry the mass. In the case of the present invention this moderate heating also serves to cure the binder by forming a cross-linked polymer. Such nonwoven fabrics have wet strength values as great as the usual woven cotton fabrics. These fabrics have the outstanding advantage of low cost, both in comparison with woven fabrics and with nonwoven fabrics prepared in efforts to provide comparable properties with binders previously available. Polyvinyl acetate is an extremely low cost binder and its use as the major component of the binders of the present invention provides a highly significant cost advantage over other types of binders which have been proposed for providing textile-like qualities in nonwoven fabrics.

External plasticizers or external curing agents are not needed with the binders of this invention, which greatly However, either or both can be used to modify the properties in special cases. Thus some external plasticizer can be added when an extremely soft fabric is desired. Also, it has been found that, even though the binders; of this invention are post-curable and provide unusually strong fabrics, external curing agents may be used in combination therewith to further enhance the strength of the resulting fabric.

Useful criteria for comparing the effectiveness of binders are obtained by evaluating fabrics, prepared with the binders by similar procedures, for wet break strength, the ratio of wet break strength divided by dry break strength, and the ratio of wet break strength to stifiness. High values of all three parameters are, of course, desirable. In accordance with this invention, nonwoven fabrics are readily prepared with 20% to 30% binder which have wet break strengths of about 3 to 4 lbs/inch/oz/sq. yard, ratios of wet/ dry break strengths of about 0.7 to 1.0, and ratios of wet break strength/stiffness (measured as described subsequently) of 0.4 to 0.8. All of these values are surprisingly better than were previously possible with polyvinyl acetate binders, including plasticized polyvinyl acetate or copolymers of vinyl acetate having external curing agents.

The following examples, wherein parts and percentages are by weight, illustrate specific embodiments of the invention:

EXAMPLE 1 An aqueous dispersion of 69 parts by weight of vinyl acetate, parts dibutyl maleate, and 1 part glycidyl methacrylate in 200 parts of water is prepared with 1 part of dodecyl trimethyl ammonium chloride cationic dispersing agent. The mixture of monomers is polymerized at 50 C. in the presence of 0.05 part of an initiatoractivator catalyst system consisting of hydrogen peroxide and zinc formaldehyde sulfoxalate to form a aqueous dispersion of a vinyl acetate/dibutyl maleate/glycidyl methacrylate interpolymer. The polymerization is rapid, requiring only slightly longer than preparation of polyvinyl acetate homopolymer under the same conditions. A high molecular weight interpolymer is obtained which is not crosslinked (it is soluble in acetone) but which will crosslink upon heating to give highly desirable binder properties. In a similar way aqueous dispersions are prepared by interpolymerizing the following mixtures of monomers in aqueous dispersion:

(a) 48 parts vinyl acetate/48 parts dibutyl maieate/S parts glycidyl methacrylate,

(I7) parts vinyl acetate/20 parts dibutyl fumarate/3 parts N-rnethylol acrylamide,

(c) parts vinyl acetate/ 15 parts dibutyl fumarate/ 1 part allyl glycidyl ether,

(d) 85 parts vinyl acetate/ 15 parts dibutyl fumarate/l part glycidyl methacrylate, and

(e) 76 parts vinyl acetate/22 parts of ethyl acrylate/2 parts glycidyl acrylate.

The copolymer binders of this invention can also be prepared in other ways, as will be obvious to those skilled in the art of vinyl polymerizations. For example, the monomers can be added to an aqueous solution of the dispersing agent and catalyst held at 60 -80 C. The dispersion polymerization can be carried out continuously by introducing dispersions of the monomers, dispersing agent and catalyst into a reactor maintained at 6080 C.

When hydroxyethyl polyvinyl alcohol or polyvinyl alcohol are used as dispersing agents in amounts of 1% to 5%, based on the total weight of dispersion, the aqueous emulsion of the interpolymer is especially suitable for direct use in impregnating nonwoven webs with binder. It is usually sufiicient to saturate the nonwoven material with the emulsion, squeeze out the excess, and then dry in hot air at 150 C. to remove the moisture and cure the binder. The simplicity of this method of application is a greater advantage over other types of binders which have'required special methods of application in order to avoid serious migration of binder during drying.

When migration is encountered with the use of other disperser-s, it can be overcome by coagulating or precipitating the binder on the fibers after impregnating the Web, either before or during the drying step. The web can be saturated with the aqueous dispersion of binder and the binder then coagulated by immersion in an aqueous coagulating bath, containing acid, alkali or salts such as sodium chloride or aluminum sulfate. Conditions which will accomplish the desired result without damaging the fibers of the web will be obvious to one skilled in the art. The web is then rinsed, as by dipping in water, to remove acid, alkali or salt, excess water is squeezed from the web and the web is dried. Another method is to apply the binder in an aqueous solution of methanol, acetone or other Water-miscible solvent, in which case the coagulating bath can simply be water and no further rinsing is needed.

EXAMPLE 2 Three grams of hydroxyethyl polyvinyl alcohol (29% ethylene oxide content) are dissolved in 50 mls. 0.1 molar potassium dihydrogen phosphate, 20 mls. 0.1 N sodium hydroxide, and 30 mls. water. In this cold solution is dissolved 0.2 gm. sodium lauryl sulfate, 0.2 gm. potassium persulfate, and 0.1 gm. sodium bisulfite. The solution is warmed and 70 gms. vinyl acetate, 30 grns. dibutyl maleate, and 1 gm. glycidyl methacrylate monomers are added to the solution at 7073.5 C. over a period of 1.5 hours. The polymerization is allowed to proceed to completion over another 2 hour period. A very smooth, viscous dispersion of binder results, with essentially no coagulation present.

A random-laid web of scoured and bleached Egyptian cotton (A-A filter grade, 1 /2 inch fiber length) is prepared which weighs about 2.2 ounces per square yard, and which is highly homogeneous so that there is little difference between the length and width directions as to dry tensile, wet tensile and percent elongation. The web is saturated with a 10% aqueous dispersion of the binder. The web takes up about 8 times its weight of the aqueous dispersion. The saturated web is passed through a wringer under a controlled loading and adjusted to leave about 30% by weight of binder solids in the web. The wet Web is hung in a circulating air oven at -135 C. for 15 minutes to dry and cure. The results of physical tests of the resulting fabric are shown in Table I.

In preparation for physical testing the Web is stored at 65% relative humidity and 70 F. for 48 hours to provide standard moisture conditions. Break strength and elongation are suitably determined on an Instron tester, using 1 inch x 6% inch web samples placed with 2 inches between the grips, with a crosshead speed of 1 inch per minute. The average of 6 to 12 determinations is used to obtain characteristic values. Wet break strengths are determined in a similar manner with samples which are soaked in water for 30 minutes immediately prior to testing.

Tear strength is determined with 3-inch x 6-inch samples. A trapezoid is marked off with parallel sides 1 and 4 inches long, the corresponding oblique sides being 3% inches. The sample is placed in the Instron tester with the oblique sides of the trapezoid aligned with the faces of the grips. The sample is cut inch on the 1- inch side of the trapezoid to start the tear. The average load at a crosshead speed of 2 inches per minute is taken as the tearing strength.

Stiffness is determined by the single cantilever method at the angle formed by an inclined plane just touching the edge of a horizontal surface to form an angle of 41.5 A fabric sample 1 inch wide is held fiat on the horizontal surface with the end extending over the inclined plane for the minimum distance required for the sample to sag suiiiciently to touch the inclined plane. This distance in inches is taken as a direct measure of the stiffness or flexibility.

Table l V V v [Eirect of various vinyl acetate binders on properties of nonwgven fahrifs containing 30% of hinder applied by padding from aqueous ispersion Additional Binder Components Wet Ratio, Ratio, Binder Strength Wet/Dry Wet (lbs.lin./ Strength Strength] Plasticizer Curing Catalyst Curing Agent oz./sq. yd.) Stiffness (1) 69 V. Ac./30 Di Bu Male- 12% Dibutoxy- 0.3% Zinc 2% Dimethylol 3.6 0.9 0. 7

ate/1 Glycidyl Methacryethyl phthalate. fluoborate. ethylene urea. late. (2) 69 V. Arr/30 Di Bu Maledo .do 6% Dimethyiol 4.1 0.91.0 0.6

1ate/1 Glycidyl Methacryethylene urea.

ate. (3) 69 V. Ac./30 Di Bu Male- .do do 2% Glutaralde- 3.9 1.0 0.7

ate/1 Glyeidyl Methacryhyde. I ate. (4) 48 V. Ae./48.Di Bu Male- None do 2% Dimethylol 3. 7 0.9 0. 5

ate/5 Glyeidyl Methaeryethylene urea. ate. (5 Polyvinyl Ace o 7 -Nnm= N 0. 4 0.1 0.1 (6i Polyvinyl Acetate Dibntoxy- 013% Zinc p 2% Trimethylol 1.1 0.7 0. 2

ethyl phthalate. tiuohorater melamine resin. (7.) 80 V. Ac./ Ethyl acrylate. 12% Dibutoxy- None None 2.5 0.4 0.4

ethyl phthalate. (8) 85 V. Ac.l15 Octyl aerylate. 15% Dibutoxy- 0.3% Zinc 2% Dimethylol 2. 5 0.8 0. 4

ethyl phthalate. fluoborate. ethylene urea.

Table 1 includes data for comparison on the properties of nonwovcn fabrics obtained with polyvinyl acetate hom opolymer binder, both with and without added external plasticizer and curing agent, and with oopolymers of vinyl acetate and ethyl, acrylate or ootyl acrylate. It will be seen that polyvinyl acetate alone (item 5) gave quite poor results, .and that the addition of external plasticizer and curing agent (item 6) gave somewhat better results but which were nevertheless poor in comparison with the results obtained with the terpolymer binders of the present invention. 1 The terpolymers also gave much better results than were obtained with the vinyl acetate copolyrners (items 7, 8) Similar results are obtained when the binder is applied by coagulation from solution.

EXAMPLE 3 Nonwoven fabrics are prepared to contain about 20% binder. The binder, which is prepared by the polymerization procedure described in Example 1, is applied from solution in acetone and coagulated in the web by dilution with water. The results of physical tests of the Theexternal curing agents found effective in supplementing the effect of the internal curing agent may be classified as follows:

(a) For terpolymers containing 0.5 5% epoxide groups, use of 0.5-14% formaldehyde-donors such -as urea-formaldehyde, melamine-formaldehyde, or triazoneformaldehyde reaction products or prepolymers such as dimethylol urea, trimethylolmelamine, dimethylolethyleneurea, and a commercial product defined as butylated melamine-formaldehyde resin. Dialdehydes such as glyoxal, glutaraldehyde, a-hydroxyadipaldehyde, etc., are also effective. An acid-generating activator for the formaldehyde donor or the dialdehyde, such as zinc fluoborate, zinc chloride, or butyl phosphoric acid, is useful in trace amounts (QM-0.3%) for accelerating the cure.

(b) For terpolymers containing 2-8% methylol groups, addition of 112% epoxy compounds, such as dipentene dioxide, diglycidyl ether of glycerol or diglycidyl ether of diphenylolpropane, and epoxy esters of various types, such as the esters of epoxyhexahydrophthalates, epoxy tall oil fatty acids, or epoxy oleic acid. An acid accelerresulting fabric are shown in Table II. ator 1s also advantageous in these cases.

Table II [Efiect of various vinyl acetate binders on properties of nonwoytren iabrics containing 20% of binder applied by coagulation from ace one Additional Binder Components Wet Ratio, Ratio, Binder Strength Wet/Dry Wet (lbs./in./ Strength Strength] Plasticizer Curing Catalyst Curing Agent o7../sq. yd.) Stifiness 80 V. Ac./20 Di Bu Fumarate/3 None None N 3.0 0. 8 0.3

N-Mcthylolacrylamide. 80 V. Ae.lz0 Di Bu Fumarate/3 12% Dibutoxyn-Butyl phosphor- 1% Diepoxy 3. 2 0.0 0. 7

N-Mothylolacryla-mide ethyl phthalate. ic acid. glycerol. (38% binder in fabric). 85 V. Ae./l5 Di Bu Fumarate/l ....do ..do 0.6% Butylated 2. 9 0.7 0. 4

Allyl glycidyl ether. melamine-tormaldehyde resin. 85 V. Ac./l5 Di Bu Fumarate/l do ..d0 do 4.1 1. 0 0. 8

Glyeidyl methacrylate. 87 V. Ac./13 V. Stearate None N n N n 0. 9 0. 6 0, 1 85 V. Ac./15 Di Bu Maleate..- .dn r1n .do 0.6 0. 3 0. 1 80 V. Ac./20 Di Bu Fnmaratedo do do 0.9 0.4 0.2 87 V. Ac./13 V. Pelargonate..- do do do 1.3 0.5 0.2

It has been observed that the flexibility of the fabric can be increased by the addition of hydrophobic external plasticizer to the binder composition without loss of the advantages in wet strength described in the use of the terpolymers of this invention. This may be seen in the illustrations of the tables. Examples of external plasticizers which have proved eifective include dibutoxyethylphthal ate, dibutyl phthalate, rtricresyl phosphate, and low molecular weight polyesters known to the trade as polymeric plasticizers.

(c) For terpolymers containing 0.5 -5% carboxyl groups, a combination of the above-mentioned formaldehyde donors, epoxy compounds and mineral acid ascelerator, in 1-20% concentrations based on total binder weight, were found to be most effective.

It should be pointed out, however, that the particular external curing compositions, or their concentrations described herewith, in no way should limit the use of these terpolymers as binders, as claimed in this invention. Also, these external components may be added just bei fore application (if their stability in the dispersion or solution is not marked) or may be formulated into the aqueous dispersion of the binder and stored indefinitely if the stability in aqueous dispersion is high. The former alternative might be acceptable in many use applications, if a very rapid, low temperature cure is desired.

I have found that, for the curing compositions described in these examples, a combination drying-curing operation at 135 C. for minutes is adequate. An alternative procedure is to dry the fabrics at about 105 C., and then to cure the binder by exposure to 150 C. for 2.5 minutes. A cure at 170 C. is possible for short periods without degrading the fabric. Lower temperatures for longer exposure times have also given satisfactory results.

I also wish to point out that fibers other than the Egyptian cotton used in these examples may be applied with great advantage, particularly in lowering costs. Thus, shorter length cotton, such as our more common domestic varieties, cotton linters or rayon may be used. Also blends of these low-cost fibers with synthetic fibers such as the polyamides, polyesters, polyvinyl alcohol, polypropylene may be used to obtain special fabric effects.

Since many different embodiments of the invention may be made without departing from the spirit and scope thereof, it is to be understood that the invention is not limited by the specific illustrations except to the extent defined in the following claims.

I claim:

1. A nonwoven fabric formed of a loosely assembled web of fibers bonded together with a binder comprising at least 70% by Weight of an interpolymer consisting essentially of at least 45% by weight of vinyl acetate, 0.3% to 12% by weight of a member selected from the group consisting of N-methylol acrylamide, glycidyl acrylate, glycidyl methacrylate and allyl glycidyl ether, and at least 10% by weight of a member selected from the group consisting of compounds represented by the for-- mulas,

, 8 and CH-OOOR ll OH-CO OR,

wherein R is an alkyl group of 4 to 18 carbon atoms, said interpolymer being in the cured, cross-linked state in the fabric thereby providing high ratios of wet strength to dry strength and to stiffness.

2. In the process of preparing a nonwoven fabric from a loosely assembled mass of fibers wherein fibers are bonded together by applying a binder in a volatile liquid and then heating to remove the liquid, the improvement for preparing relatively low-cost nonwoven fabrics of enhanced properties which comprises applying in a volatile liquid a binder of an interpolymer consisting essentially of at least by Weight of vinyl acetate, 0.3 to 12% by weight of a member selected from the group consisting of N-methylol acrylamide, glycidyl acrylate, glycidyl methacrylate and allyl glycidyl ether, and at least 10% by weight of a member selected from the group consisting of compounds represented by the formulas,

wherein R is an alkyl group of 4 to 18 carbon atoms, and then heating in hot air at -150 C. to remove the liquid and cure the binder.

References Cited in the file of this patent UNITED STATES PATENTS 2,570,253 Lundquist Oct. 9, 1951 2,698,574 Dougherty et al Jan. 4, 1955 2,760,884- Graf Aug. 28, 1956 2,763,578 Simons Sept. 18, 1956 2,861,060 GOOdB NOV. 18, 1958 

1. A NONWOVEN FABRIC FORMED OF A LOOSELY ASSEMBLED WEB OF FIBERS BONDED TOGETHER WITH A BINDER COMPRISNG AT LEAST 70% BY WEIGHT OF N INTERPOLYMER CONSISTING ESSENTIALLY OF AT LEAST 45% BY WEIGHT OF VINYL ACETATE, 0.3% TO 12% BY WEIGHT OF A MEMBER SELECTED FROM THE GROUP CONSISTING OF N-METHYLOL ACRYLAMIDE, GLYCIDYL ETHEER, ARCYLATE, GLYCIDYL METHACRYLATE AND ALLYL GLYCIDYL ETHER, AND AT LEAST 10% BY WEIGHT OF A MEMBER SELECTED FROM THAT GROUP CONSISTING OF COMPOUNDS REPRESENTED BY THE FORMULAS, 